1. Field of the Invention
This invention relates to an axial piston machine with a rotating cylindrical drum, which has a multiplicity of cylindrical bores arranged concentrically to the axis of rotation with pistons capable of sliding longitudinally therein. The drum lies against a control surface adjacent the housing., in which case the pistons are in contact with a working surface that can be positioned diagonally to the axis of rotation and the cylindrical bores are provided with connecting openings to the control channels of the control surface, whereby the cylindrical drum is also supported in a longitudinally moveable manner. More particularly, the invention relates to such a machine wherein additional means is provided for controlling the pressing force of the cylindrical drum on the control surface, which is in working connection with the cylindrical drum.
2. Description of the Art
When such machines are used as pumps, especially as self-priming pumps, it is desirable that the pump have as high a suction rate as possible in order to minimize the filling losses caused by flow losses on the suction side and the resulting reduction in the delivery volume. The flow losses are a function of the flow velocity of the flow medium in the suction channel of the pump and the design of the latter. It is therefore necessary that the suction channel have as large a cross section as possible to keep the flow velocity and thus the pipe friction and flow losses low.
In the case of axial piston machines, there is a peculiarity that also leads to filling losses. This consists in the structurally induced gap between the housing control surface, which contains the orifices of the suction channel, designed as kidney-shaped control channels, and the rotating cylindrical drum lying against them. The gap is necessary so that a hydrostatic film of lubrication can form between the webs of the control surface and those of the cylindrical drum, which reduces the friction and facilitates disturbance-free and easy running of the machine. However, external leakage streams are caused by the gap, as well as inner leakage streams that flow directly from the high-pressure side to the low-pressure side of the control surface. The loss stream that results reduces the volumetric efficiency of the machine and thus the actual delivery volume. In order to achieve a satisfactory volumetric efficiency, the gap between the rotating cylindrical drum and the control surface must be kept as small as possible. This is achieved by a pressure spring which presses the cylindrical drum against the control surface and also by a connection which is produced between the cylindrical bores and the control channels through openings whose diameter is smaller than the diameter of the cylindrical bores. Through the latter measure, the cylindrical drum is pressed into the cylindrical bores against the control surface due to the fluid pressure, in which case the contact pressure is proportional to the loading of the machine. The leakage streams and/or filling losses of the axial piston machine are thus held at a low level.
The narrowed connection openings between the cylindrical bores and the control channels represent a constriction of the suction channel, which leads in the case of a certain desired delivery volume to a certain flow velocity in this region and thus to flow losses and, in accordance with the flow rate, to a restriction in the suction power of such a machine versus a machine lacking this structural means.
The geometric relationships on the suction side are thus essentially prescribed by the power and moment equilibrium between the hydrostatic release of the rotating cylindrical drum and limitation of the relief gap.
An axial piston machine of swash plate construction is disclosed in German patent DE-OS 22 50 510, in which an additional device situated around the outside diameter of the cylindrical drum induces an increase in the pressing force of the cylindrical drum on the control surface. A disadvantage in this device, however, is that it increases the structural volume of the axial piston machine substantially and leads to a complicated and expensive construction.
The present invention proposes to avoid those shortcomings and increase the suction ability of an axial piston machine in an economical manner.